The ability to scan people, luggage, cargo etc. for the presence of metallic objects is becoming ever more important, in particular at airports and other locations with restricted access.
Conventional metal detectors are based on pulse induction (PI). Typical PI systems use a coil of wire on one side of a portal (usually an arch) as the transmitter and receiver. Short bursts (pulses) of current are sent through the coil of wire, each pulse generating a brief magnetic field. When a pulse ends, the magnetic field reverses polarity and collapses causing a current to run through the coil. If a metallic object passes through the metal detector, a magnetic field is induced in the metallic object, and this magnetic field interferes with the current generated in the coil. By monitoring the generated current, the metal detector can determine if a metallic object is present.
Such systems can typically only detect that a metallic object is present and don't usually give any indication as to where the object may be located. One disadvantage of this system is thus that it is generally necessary to scan a single individual at any one time. This is because if two people were to be scanned at the same time, it would not be possible to determine who was carrying the metallic object. This can mean that such systems are not suitable for some applications, such as at railway and subway stations or other mass transit hubs, as to provide the desired throughput of individuals would require an impractical number of portals and associated screening staff.
It is therefore desirable to provide a monitoring system that is able to scan multiple objects at a time, while still accurately being able to detect metallic objects, in order to increase throughput at security checkpoints. It is further desirable to provide a monitoring system that can provide an indication as to the location of a detected metallic object.